The present invention relates to a monomode fiber reciprocal ring interferometer device.
In a ring interferometer, or SAGNAC interferometer, two beams travel in opposite directions over the same optical path and interfere at the output of this path. As long as a disturbance of this path has the same characteristics for both directions of propagation and does not vary during the whole of the transit time of the light in the interferometer, the two beams are affected identically and their relative phase remains unchanged. Disturbances of this type are called "reciprocal". Because the transit time in an interferometer is generally very small, the variations of the disturbance during this time, unless it is introduced voluntarily, are generally negligible.
But there exist "non reciprocal" disturbances which have a different amplitude in the two directions of propagation, these are physical effects which, by establishing its complete orientation, destroy the symmetry of the space or of the medium.
Two known effects present this property:
the Faraday effect, or colinear magneto-optical effect, by which a magnetic field creates a preferential orientation of the spin of the electrons of the optical material;
and the Sagnac effect, or relativistic inertial effect, in which the rotation of the interferometer with respect to a Gallilean reference destroys the symmetry of the propagation times.
The use of rotation with respect to the inertial space leads to constructing optical fiber gyrometers and use of the magnetic field leads to the construction of current sensors or magnetometers.
It has been shown that the use of a particular so called reciprocal configuration exactly cancels out any phase shift other than those induced by the non reciprocal effects.
With this configuration however it is necessary to detect the light returning through the monomode input gate of the interferometer. This requires using a separator which feeds a part of the outgoing light to a detector while coupling input light. The return signal to the detector is optimum when the separator is of the 50--50 type.
In actual fact, it is a question of a second separator for a ring interferometer requires a first separator or beam divider at the inlet of the ring, injecting therein two contrarotating waves and recombining them after they have travelled round the ring.
In addition to the use of conventional optical components, it has been proposed to use components used in guided optics: integrated optics or all fiber.
The separators are generally formed more particularly using integrated optics. Now the two separators do not play an equivalent role. The first separator must have at least three monomode gates so as to be efficiently coupled to a unimode input-output filter and the two ends of the monomode fiber coil of the ring. On the other hand only two monomode gates are required on the second separator: source or its priming fiber and unimode input-output filters. The gate opening into the detector does not need to be monomode without the interferometer ceasing to be monomode for all that.
It is therefore possible to simplify the architecture of a reciprocal ring interferometer and this is what the invention provides.